2.1 Field of the Invention
The present invention relates to an anode made of a graphite material capable of intercalating and de-intercalating lithium ions, and a nonaqueous electrolyte secondary battery using the same.
2.2 Description of the Prior Art
In the recent trend of rapid progress in portable and cordless structure of consumer electronic appliances, the lithium secondary battery is noticed as their driving power source.
Hitherto, as the materials for anode of lithium secondary battery, lithium metal, lithium alloy, and carbon capable of intercalating and de-intercalating lithium ions have been discussed, but the former two materials produce dendritic lithium or fine lithium alloy respectively along with process of charging and discharging of battery, possibly leading to internal short-circuit of battery. Recently, therefore, the lithium ion battery using carbon is in the mainstream of merchandise.
When carbon is used as the anode, since lithium is intercalated between carbon layers at the time of charging, lithium is not present on the anode surface in metal state, and it is hence said that the safety of the battery may be enhanced.
Among carbon, graphite is particularly small in the initial irreversible capacity, and is likely to raise the electrode density, and it is studied in various aspects.
Such graphite materials include natural graphite, and artificial graphite obtained by calcining pitch, coke or other organic material. Generally, graphite particles are composed as polycrystals of graphite crystallites with crystallite size ranging from several nm to hundreds of nm in the in-plane direction ((110) or (100) direction) or C-axis direction ((004) or (002) direction). In such graphite particles, the C-axis of crystallites tends to face nearly same direction, and the same tendency is noted in the particles after grinding and sieving. Accordingly, the in-plane direction and C-axis direction are respectively uniform as if entire graphite particles were one crystallite.
When grinding the graphite in order to reduce the particle size, the graphite is likely to be cleaved by the shearing force between layers, that is, in the in-plane direction of crystal. Usually, therefore, the graphite particles ground to particle size of scores of microns are shaped like scales, the particle size is small in the C-axis direction of crystallite, and the aspect ratio of particle size in the in-plane direction of crystallite and particle size of C-axis direction tends to be larger.
Using such graphite material as the anode material, when paste is prepared together with binder and others and applied and rolled on the current collector, the filling density of graphite material in the electrode is raised, and owing to the large aspect ratio of particles in the in-plane direction and C-axis direction, the C-axis direction of particles tends to coincide with the vertical direction of the current collector. That is, the basement surface of crystallite in the graphite particles (C-axis (004) or (002) direction) tends to orient in the same direction as the surface of the current collector.
The orientation of graphite material in the electrode can be known from the peak intensity ratio R of the diffraction line (110) in the in-plane direction obtained from the wide-angle X-ray diffraction and the diffraction line (004) in the C-axis direction.   R  =                    (        110        )            ⁢              xe2x80x83            ⁢      peak      ⁢              xe2x80x83            ⁢      integral      ⁢              xe2x80x83            ⁢      intensity      ⁢              xe2x80x83            ⁢      I      ⁢              xe2x80x83            ⁢              (        110        )                            (        004        )            ⁢              xe2x80x83            ⁢      peak      ⁢              xe2x80x83            ⁢      integral      ⁢              xe2x80x83            ⁢      intensity      ⁢              xe2x80x83            ⁢      I      ⁢              xe2x80x83            ⁢              (        004        )            
The intensity ratio R of graphite material measured in the powder state before application is measured in a state in which each particle does not have orientation in the measuring surface of wide angle X-ray diffraction, and therefore the obtained value corresponds to the size ratio of crystal size in the in-plane direction of graphite material and crystal size in C-axis direction. By contrast, in the electrode prepared by applying and rolling paste compound of graphite material on the current collector, the basement surface of graphite particles tends to orient in the same direction as the current collector surface. Therefore, crystallites composing graphite particles also orient according to the orientation of particles, and when the electrode surface is measured by X-ray, as compared with the powder state before application, the peak intensity I (110) of the in-plane direction of crystallites is weak, and the peak intensity I (004) in the C-axis direction is strong, so that the peak intensity ratio R varies. Thus, from the change in the peak intensity ratio R of wide angle X-ray diffraction, the degree of orientation of particles in the electrode may be known.
When the conventional electrode was measured in the above method, the peak intensity ratio R was about 0.01 to 0.05, and the ratio P(=R/Ro) of R to the peak intensity ratio Ro obtained from the powder before preparation of electrode was about 0.05.
In such electrode, on the electrode surface at the interface to the electrolyte, the ratio of existence of the basement surface of graphite crystal is large, while the ratio of existence of edge of graphite crystal inducing intercalation of lithium ions is small. Hence, in charging and discharging reaction, lithium ions cannot move smoothly at the interface of electrolyte and electrode, and polarization is likely to occur, and therefore favorable high rate charging and discharging characteristic or charging and discharging cycle characteristic cannot be obtained.
To solve such problems, as disclosed in Japanese Laid-open Patent No. 4-190556, Japanese Laid-open Patent No. 4-190557, and Japanese Laid-open Patent No. 6-318459, for example, it has been proposed to reduce the crystal size ratio (aspect ratio) in the in-plane direction and C-axis direction of graphite crystallites. In spite of these proposals, however, the problems are not solved completely, and in particular nothing is considered about restriction of orientation of graphite particles on the electrode.
In Japanese Laid-open Patent No. 8-83609 or Japanese Laid-open Patent No. 8-180873, graphites having various particle shapes are proposed, but nothing is still considered about restriction of orientation of graphite particles on the electrode.
The invention is devised to solve these problems, and it is hence an object thereof to present a nonaqueous electrolyte secondary battery using an anode particularly excellent in high rate discharging characteristic and charging and discharging cycle characteristic.
The object of the invention is to present a nonaqueous electrolyte secondary battery using an anode particularly excellent in high rate discharging characteristic and charging and discharging cycle characteristic. To achieve such object, as the anode for nonaqueous electrolyte secondary battery, the invention uses an anode mainly composed of a graphite material, with its peak intensity ratio R (=I(110)/I(004)) ranging from 0.05 to 0.5. As a result, it prevents extreme parallel orientation of crystal layer of graphite material on the current collector to the current collector plane, and enhances the high rate discharging characteristic.
Also in the anode, the ratio P (=R/Ro) of peak intensity ratio R obtained from the electrode prepared by applying and rolling a graphite material on a current collector and peak intensity ratio Ro obtained from the powder before preparation of the electrode is in a range of 0.1 to 0.7.
It hence controls extreme orientation of graphite particles in the electrode in the electrode preparation process, and enhances the high rate discharging characteristic.
By using such anode, a nonaqueous electrolyte secondary battery excellent in high rate discharging characteristic is obtained.
The invention as set forth in claim 1 uses an electrode of which peak intensity ratio R (=I(110)/I(004)) of lattice planes (110) and (004) of graphite material obtained by wide angle X-ray diffraction of the anode for nonaqueous electrolyte secondary battery is in a range of 0.05 to 0.5.
In the anode using the graphite material of which peak intensity ratio R is in a range of 0.05 to 0.5, on the electrode surface at the interface to the electrolyte, the basement surface and edge of graphite crystals coexist adequately. Such anode is easy to manufacture, especially when graphite particles are in spheroidal or massive shape. The graphite of such particle shape is, as compared with the flake graphite, smaller in the aspect ratio of the direction corresponding to the in-plane direction of particle and the direction corresponding to the C-axis direction, and if exposed to pressure in rolling process when fabricating the electrode, the basement surface of each particle is hardly oriented uniformly in the same direction as the current collector surface. As a result, many edges of graphite crystal are present on the electrode surface. Even in the flake graphite, by adjusting the deposition condition or rolling process condition at the time of manufacture of electrode, orientation of particles in same direction can be suppressed, and it seems possible to manufacture.
In this anode, since there are adequate edges of graphite crystals on the electrode surface at the interface to the electrolyte, intercalation of lithium proceeds smoothly, and polarization during charging and discharging is suppressed, and an electrode excellent in high rate discharging characteristic is composed. Also because the move of lithium in the anode is smooth, all parts of the anode react uniformly, and deterioration is small if charging and discharging are repeated. Moreover, volume expansion or shrinkage of graphite material due to intercalation and de-intercalation of lithium ions is not specified in one direction only, and deterioration such as dropout of compound from the electrode in the charging and discharging cycles is suppressed, and an electrode excellent in cycle characteristic is composed.
By contrast, in the anode of which peak intensity ratio is less than 0.05, much basement surface of graphite crystal is present on the electrode surface at the interface to the electrolyte. Therefore, only few edges of crystals are present, and intercalation of lithium is not smooth, and polarization increase, and favorable high rate discharging characteristic and cycle characteristic are not obtained.
On the other hand, when the peak intensity ratio exceeds 0.5, the basement surface of graphite particles does not orient in a specific direction, but is present uniformly in all directions in the electrode, and electron conduction by contact of graphite particles is not obtained sufficiently, and polarization occurs, which is not preferred, too.
The invention as set forth in claim 2 is to present a nonaqueous electrolyte secondary battery of high voltage, large capacity, and excellent in high rate discharging characteristic and cycle characteristic, by combining the anode of claim 1, a cathode composed of lithiated complex oxide, and a nonaqueous electrolyte.
The invention as set forth in claim 3 relates to the nonaqueous electrolyte secondary battery of claim 2, in which the solvent of the nonaqueous electrolyte is mainly composed of two components, cyclic carbonate and chain carbonate, or three components further including aliphatic ester carbonate.
When the lithiated complex oxide is used in the cathode, the potential of the cathode is about 4 V as compared with the potential of lithium, and this is the potential for oxidizing and decomposing most organic solvents. By using the electrolyte existing stably even at such high potential, existing stably as liquid in a high temperature range, and holding a high electric conductivity, it is intended to present the nonaqueous electrolyte secondary battery further excellent in low temperature characteristic and storage characteristic.
The invention as set forth in claim 4 relates to the nonaqueous electrolyte secondary battery of claim 2, in which the cathode and anode include an organic electrolyte and a polymer for absorbing and holding the organic electrolyte, and the separator includes an organic electrolyte and a polymer for absorbing and holding the organic electrolyte, same as in the cathode and anode. In such battery composition, a battery of high performance and flexible shape is realized.
The invention as set forth in claim 5 is an anode, in which if the peak intensity ratio of lattice planes (110) and (004) of graphite material obtained by wide angle X-ray diffraction is R (=I(110)/I(004)), the ratio P (=R/Ro) of measurement R of the anode formed by applying and rolling graphite material on an current collector, and measurement Ro of the powder before fabrication of electrode is in a range of 0.1 to 0.7.
The electrode of which ratio P is in a range of 0.1 to 0.7 is relatively smaller in the change of R as compared with that of the powder before fabrication of electrode. It is likely to settle in this range when the graphite material is particles in spheroidal or bulk shape. This is because the aspect ratio is smaller as compared with the flake graphite, and if exposed to pressure in the rolling process in fabrication of electrode, the basement surface of each particle is hardly oriented in the same direction as the current collector surface. Or, if the graphite material is scaly particles large in aspect ratio and likely to orient, orientation of particles in same direction may be suppressed by the deposition condition or rolling process condition in electrode fabrication. In such electrode, crystallites are not oriented in a specific direction, and many edges of graphite crystals are present on the electrode surface, and lithium ions are likely to be intercalated, and moving in the electrode is considered to be smooth. As a result, favorable high rate discharging characteristic and favorable cycle characteristic may be obtained. Moreover, since expansion and shrinkage of graphite materials due to intercalation and de-intercalation of lithium ions are not specified in one direction only, deterioration of electrode strength such as dropout of compound due to repeated charging and discharging can be suppressed, and an electrode excellent in cycle characteristic is composed.
In the electrode of which ratio P is less than 0.1, the orientation of graphite particles differs significantly before and after electrode fabrication, and in the graphite particles after electrode fabrication, the basement surface of crystals is oriented in the same direction as the current collector surface. Accordingly, at the interface to the electrolyte, intercalation of lithium ions is not smooth, and polarization is likely to occur, and favorable high rate discharging characteristic and cycle characteristic are not obtained.
In the electrode of which ratio P exceeds 0.7, as compared with the powder before electrode fabrication, although the change of R is smaller, the rolling process is not enough, and the filling density of graphite is low. Hence, the contact between particles is insufficient, and sufficient electron conduction is not achieved, and polarization increases to lower the initial capacity, which is not preferred. Therefore, the ratio P must be 0.1 or more and 0.7 or less, and more preferably 0.2 or more to 0.5 or less.
The invention as set forth in claim 6 is to present a nonaqueous electrolyte secondary battery of high voltage, large capacity, and excellent in high rate discharging characteristic and cycle characteristic, by combining the anode of claim 5, a cathode composed of lithiated complex oxide, and a nonaqueous electrolyte.
The invention as set forth in claim 7 relates to the nonaqueous electrolyte secondary battery of claim 6, in which the solvent of the nonaqueous electrolyte is mainly composed of two components, cyclic carbonate and chain carbonate, or three components further including aliphatic ester carbonate. When the lithiated complex oxide is used in the cathode, the potential of the cathode is about 4 V as compared with the potential of lithium, and this is the potential for oxidizing and decomposing most organic solvents. By using the electrolyte existing stably even at such high potential, existing stably as liquid in a high temperature range, and holding a high electric conductivity, it is intended to present the nonaqueous electrolyte secondary battery further excellent in low temperature characteristic and storage characteristic.
The invention as set forth in claim 8 relates to the nonaqueous electrolyte secondary battery of claim 6, in which the cathode and anode include an organic electrolyte and a polymer for absorbing and holding the organic electrolyte, and the separator includes an organic electrolyte and a polymer for absorbing and holding the organic electrolyte, same as in the cathode and anode. In such battery composition, a battery of high performance and flexible shape is realized.
The graphite material used in the anode is not particularly limited, and includes, for example, ground and sieved natural graphite, artificial graphite obtained by calcining pitch, coke or other organic material, mixing with binder pitch, forming, and graphitizing at 2000 to 3000xc2x0 C., being ground and sieved into bulk or scale particle shape, or spheroidal graphite obtained by graphitizing the spherulite obtained from meso-phase pitch. Incidentally, also in the condition heightened in the filling density when fabricating the electrode, the orientation of particles can be suppressed, and for manufacturing a battery using an electrode of high filling density, spheroidal graphite or bulk graphite of which particle shape is close to cube is preferred. Further, the bulk graphite is preferred for manufacturing a battery having a high initial capacity because the graphitizing degree of graphite material is high and the reversible capacity is large.
As the graphite material, the lattice distance d002 of lattice plane (002) by wide angle X-ray diffraction is preferred to be 3.35 angstroms or more and 3.37 angstroms or less. In the graphite material exceeding 3.37 angstroms, the graphitizing degree is low, and the lithium intercalating reversible capacity drops, and higher capacity is not expected. The median diameter D50 is preferred to be 10 to 20 microns. Hence the filling density is enhanced, and the electrode excellent in coating and roller performance can be fabricated. To suppress the side reaction occurring in decomposition of electrolyte or the like on the electrode surface, the specific surface area determined by the BET adsorption method is preferred to be 2.0 to 5.0 m2/g.
The wide angle X-ray diffraction was measured by using CuK xcex1 as X-ray source by means of RINT-2500 (manufactured by RIGAKU DENKI). For measurement of electrode, a part of the electrode was cut out, and adhered to a sample holder and measured. As for graphite powder, according to the measuring method using a sample having no orientation in any direction (Guideline of X-ray Diffraction, RIGAKU DENKI, p. 42), as amorphous substance, silica gel powder was mixed in the sample by about 50%, blended and ground in an agate mortar, and charged into a sample holder and measured. As the graphite powder used at this time, the powder before fabrication of anode may be used, or the compound of electrode after fabrication may be collected, and used after sufficient separation between particles in a mortar before measurement. When measuring the wide angle X-ray diffraction of electrode and powder, the sample surface for incident X-ray is a plane, its surface coincides with the axis of rotation of goniometer, so that there is no measuring error in diffraction angle or intensity.
In the rolling process of electrode, any pressing technique may be employed, and a roller press, for example, is preferably used.
As the cathode material to be combined with the anode, any metal oxide containing lithium may be used as far as lithium can be intercalated and de-intercalated, and in particular those showing high potential of 4 V class are effective from the viewpoint of high energy density, and examples include LiCoO2, LiNiO2, and LiMn2O4.
As the organic solvent, preferably, the cyclic carbonate includes ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC), the chain carbonate includes dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), and the aliphatic ester carbonate includes methyl propionate and ethyl propionate.
Examples of the electrolyte include lithium perchlorate (LiClO4), lithium hexafluoride (LiPF6), lithium borofluoride (LiBF4), lithium arsenic hexafluoride (LiAsF6), lithium trifluoromethane sulfonic acid (LiCF3SO3), lithium bistrifluoromethyl sulfonyl imide [LiN(CF3SO2)2], and other lithium salts, which may be used either alone or in combination of several proper types, and, in particular, lithium hexafluoride (LiPF6) is preferred.
The dissolving amount of the electrolyte in the organic solvent is 0.2 to 2 mol/liter, more preferably 0.5 to 1.5 mol/liter.
The polymer for absorbing and holding the organic electrolyte in claims 4 and 8 of the invention may include a polymer of which degree of crystallization of volatile organic solvent or volatile liquid after evaporation is 0 to 60 wt. %, preferably 5 to 50 wt. %, or a polymer alloy mechanically kneaded and blended, or chemically bonded partially. Above all, it is preferred to use fluorine polymer or fluorine polymer alloy. As the polymer or polymer alloy, for example, it may be formed of at least one polymer selected from fluorine substitute of ethylene and its copolymers as components for forming a crystal phase, and at least one polymer selected from fluorine substitute of propylene and fluorine substitute having silicon in the principal chain as the component for forming an amorphous phase. Examples of the polymer for forming the crystal phase include polyvinylidene fluoride (PVDF), monofluoride ethylene polymer (PVF), polychloride trifluoride polymer (PCTFE), tetrafluoroethylene polymer (PTFE), and polyethylene (PE).
On the other hand, examples of the polymer for forming the amorphous phase include polyhexafluoropropylene (PHFP), perfluoroalkyl vinyl ether (PVE), and PVMQ (material symbol by ASTM) which is a fluorine substitute polymer containing silicon bond in principal chain. However, usable materials are not limited to these examples alone. In particular, as the polymer for holding the electrolyte, it is preferred to use a fluorine polymer obtained by copolymerization of 60 to 97 wt. % of vinylidene fluoride as the component for forming the crystal phase, and 40 to 3 wt. % of hexafluoropropylene as the component for forming the amorphous phase. In the copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP), VDF contributes to enhancement of mechanical strength in the skeleton of the copolymer, and HFP is taken into the copolymer in an amorphous state, and functions as the portion for holding the organic electrolyte and passing of lithium ions.
As the volatile organic solvent, those evaporated quickly in the film forming process, and useful for forming of binder of favorable separator layer and positive and negative electrode layers are preferred. Specific examples include ketones (for example, acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK), methyl isoamyl ketone), carbohydrates (for example, tetrahydrofuran (THF), methyl tetrahydrofuran), esters (for example, methyl acetate, ethyl acetate), dichloromethane, 1,2-dimethoxy ethane, 1,3-dioxolan, isophorone, cyclohexanone, and other nonaqueous solvents with boiling point around 100xc2x0 C. Although the boiling point is as high as 202xc2x0 C., N-methyl pyrrolidone is also effective because it has a high vapor tension, and is hence volatile and large in dissolution of the polymer. A volatile liquid having affinity for the volatile organic solvent may be any material higher in boiling point than the volatile organic solvent, larger in protonic property, and lower in melting point than the polymer. Specific examples include water, alcohols, esters, and carbonates. In particular, it is preferred to use water.
In order to enhance the impregnation of organic electrolyte into the polymer by adding such volatile liquid, the volatile liquid is preferred to be added to the volatile organic solvent in which the polymer is dissolved by 0.2 wt. % or more. As the addition of the volatile liquid increases, when the separator layer surface and fracture structure after evaporation of the volatile liquid were observed by scanning electron microscope (SEM), microporous cavities are increased, and a close correlation with the increase of impregnation of electrolyte is noted. The upper limit of the addition is preferred to be 15 wt. % of the volatile nonaqueous solvent in which the polymer is dissolved. More preferably, the addition of the volatile liquid is 0.5 wt. % to 10 wt. % of the volatile nonaqueous solvent in which the polymer is dissolved.
The separator composed of the polymer containing the organic electrolyte can be manufactured in a method of adding an organic electrolyte to a polymer mixed solution composed of a volatile organic solvent in which the polymer is dissolved, and a volatile liquid having an affinity for this volatile organic solvent, and forming a film by evaporating the volatile organic solvent and volatile liquid.
As the polymer and the volatile organic solvent, the same materials as mentioned above may be used.
The same effects are obtained regardless of the shape or size of the battery, such as cylindrical, square, or flat shape.